This invention relates to lamp monitor apparatus for determining the outage of a lamp in a lamp circuit. In particular, this invention relates to lamp monitor apparatus suitable for application to a motor vehicle, which typically includes a plurality of lamp circuits having different numbers and types of lamps, a complicated wiring harness, a possibly noisy electrical power supply and other complicating factors.
One known method of monitoring vehicle lamp circuits takes advantage of a high degree of symmetry in many of the circuits, which contain lamps in pairs of the same type operated simultaneously for the same purpose. Many vehicle lamps, such as headlamps, taillamps, stoplamps, turn signal lamps, etc. are duplicated on two sides of the vehicle. The prior art method of lamp monitoring thus provides a series length of wire, known as "ripcord", of matched impedance in the wiring harness current supply to each of a pair of lamps and compares the voltage drops across the wires, with a predetermined difference in voltage indicating a lamp outage. This method is adaptive in a sense, in that it uses each lamp as a reference for the other and will thus adapt to changes which affect each branch of the lamp circuit identically. However, it is somewhat cumbersome, expensive and easily affected by manufacturing variables. In addition, it is limited in application, since not all changes affect each branch of the lamp circuit identically.